Optical filters are widely used to control visible light and solar energy. Optical filters have found a range of use in vehicle and architectural glazings, as well as opthalmic devices. A number of technologies have been developed to vary the degree of light transmittance using photochromic, thermochromic, electrochromic, liquid crystal and suspended particle display technologies.
Transfer of electrons into or out of electrochromic layers, or oxidation or reduction of chromogenic compounds, enable control of the color or transparency of a switchable material with the application or removal of electricity. U.S. Pat. No. 4,902,108 describes a “self-erasing” electrochromic device where the molar extinction coefficients of the anodic and cathodic compounds in the solution changes with their electrochemical oxidation and reduction. These devices darken with the application of a potential difference, and clear again when the potential is removed. Reversal of polarity is not required to clear the device. U.S. Pat. No. 5,278,693 also describes a self-erasing electrochromic device comprising anodic, electrochromic compounds and cathodic, electrochromic compounds which, upon electrochemical oxidation, increase their molar extinction coefficient in the visible portion of the spectrum. In order to maintain a colored state in such devices (which may be used in some electrochromic mirrors, for example), continuous application of electricity is required. U.S. Pat. No. 5,457,564 suggests that this may be a drawback in large area light modulation applications, and teaches instead a device with surface-confined electrochromic materials. For applications where it is desirable that the inactive (or rest) state is dark or colored (to reduce solar heat gain in a vehicle or building, for example), the power needs can be a substantive drain on a battery over time; any energy savings from reduction of solar heat gain is reduced or negated by the power needed to maintain the dark state.
Some photochromic/electrochromic compounds are dark in their inactive (or rest) state, and oxidation of these compounds electrocatalytically transitions the material to a clear or faded state. The electrochemical oxidation occurring in such a system may be improved by provision of a partner cathodic species that balances the redox chemistry.